Electric cartridge fuse for small current intensities



March 31, 1970 R. A. BELCHER T 3,504,322

ELECTRIC CARTRIDGE FUSE FOR SMALL CURRENT INTENSITIES Filed Jan. 21, 1969 III m m0 TC NH WC WW H C m m RICHARD A. BELCHER BYM W ATTY.

United States Patent 3,504,322 ELECTRIC CARTRIDGE FUSE FOR SMALL CURRENT INTENSITIES Richard A. Belcher, Hampton Falls, N.H., and Paul C.

Hitchcock, Haverhill, Mass., assignors to The Chase- Shawmut Company, Newburyport, Mass.

Filed Jan. 21, 1969, Ser. No. 792,414 Int. Cl. H01h 85/04, 71/20 U.S. Cl. 337-164 5 Claims ABSTRACT OF THE DISCLOSURE A fuse for small current intensities-i.e. a fuse having current ratings ranging from a fraction of an ampere to a few amperes-capable of achieving substantial time-lags on occurrence of overload currents by means of a springbiased solder pot joint not requiring any heater winding, and capable of rapidly interrupting short-circuit currents, is made up of fewer and less expensive parts than prior art fuse having comparable performance characteristics. The short-circuit current interrupting capacity is achieved without resorting to a pulverulent arc-quenching filler.

BACKGROUND OF INVENTION The design of fuses for small current intensities becomes generally relatively complex when and if specific performance characteristics are required of the particular fuse such as, for instance, to achieve substantial time-lags on occurrence of overload currents, and to be capable of interrupting short-circuit currents sufficiently rapidly to achieve a drastic current-limiting action.

This invention relates to a time lag fuse requiring neither heating coils nor lag-blocks for establishing the thermal conditions needed to achieve the desired lag-type timecurrent characteristic, relying on the positive action of a spring biased solder-pot switching device having separable contacts for interrupting overload currents, and using fiber glass cloth sleeving for the dual purpose of insulating current-carrying conductors from one another, "and for absorbing the arc energy liberated during the process of interrupting short-circuit currents, thus dispensing with a pulverulent arc-quenching filler.

SUMMARY OF THE INVENTION Fuses embodying this invention include a tubular casing of insulating material having a left end and a right end, a left terminal cap being mounted on the left end and a right terminal cap being mounted on the right end of the casing. An oblong strip of insulating material is arranged inside said casing, substantially in a direction longitudinally thereof, and having a left end and a right end. A first length of a fusible conductor extends from said left terminal cap to said right end of said strip and is covered with a first sleeve of glass cloth, and a second length of a fusible conductor extends from said right terminal cap to said left end of said strip and is covered with a second sleeve of glass cloth. A stressed leaf spring conductively interconnects the ends of said first length of fusible conductor and of said second length of fusible conductor located adjacent said strip and completes a current path from said left terminal cap through said first length of fusible conductor and through said second length of fusible conductor to said right terminal cap. The fuse further includes solder means normally restraining said leaf spring in the stressed position thereof and releasing said leaf spring to interrupt said current path when said solder means reaches a predetermined temperature.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates on a very large scale the center portion of a fuse embodying this invention partly in longitudinal section and partly in side elevation;

FIG. 2 refers to the same structure as FIG. 1 and is drawn on a smaller scale than FIG. 1 and shows an entire fuse embodying this invention, FIG. 2 being taken along IIII of FIG. 3;

FIG. 3 shows the same structure as FIG. 2 and is a section taken along III-III of FIG. 2; and

FIG. 4 is a side elevation of the structure of FIGS. 2 and 3 seen in the direction of the arrow R of FIG. 2.

DESCRIPTION OF PREFERRED EMBODIMENTS Casing or fuse tube 1 is made of an electric insulating material, e.g. vulcanized fiber. Left and right terminal elements or terminal caps 2a, 2b are mounted on the ends of casing 1. An oblong strip 3 of electric insulating material is arranged inside of casing 1 substantially in a direction longitudinally thereof. Strip 3 has a left end 3a and a right end 312. A first fusible conductor or fusible wire 4a extends from left terminal cap or terminal element 2a to the right end 3b of strip 3 and a second fusible conductor or fusible element 4b extends from the right terminal cap or terminal element 2b to the left end 3a of insulating strip 3'. Thus a cross-over Cr of wires or fusible elements 4a, 4b is established adjacent the center of insulating strip 3. Fusible element 4a is covered with a sleeve 5a of insulating material and fusible element 4b is covered with a sleeve 5b of insulating material, thus preventing a short-circuit between wires 4a, 4b in the cross-over region Cr. Sleeves 5a, 5b are preferably of fiber glass cloth to form are energy absorbers for wires 4a, 4b on vaporization thereof under the action of shortcircuit currents, or heavy overload currents. A shown, for instance, in U.S. Patent 2,856,488 to F. J. Kozacka, issued Oct. 14, 1958 for Current-Limiting Fuses For Small Current Intensities, sleeves of woven fiber glass can be used as a sufficiently effective de-ionizing means to dispense with pulverulent arc-quenching fillers if the particular fuse structure is designed for small current intensities and small current ratings, respectively. Thus provision of a pulverulent arc-quenching filler such as, for instance, quartz sand, can be dispensed with inside of the cylindrical space bounded by casing 1. A leaf spring 7 as, for instance, a spring made of Phosphor bronze conductively interconnects the ends of wires or fusible elements 4a, 4b and thus completes a current path from left terminal element or terminal cap 2a through the first length 4a of fusible conductor and the second length 4b of fusible conductor to right terminal element or terminal cap 2b. The axially outer ends of fusible wires 4a, 4b are threaded through perforations in caps 2a, 2b to the outer surfaces thereof. Each terminal cap 2a, 2b is provided with a recess containing a pool 8a, 8b of solidified solder establishing a conductive connection between caps and wire 2a, 4a and 2b, 4b, respectively. Solder means 9 normally restrain leaf spring 7 in the stressed position thereof shown in FIG. 1 in solid lines. Solder means 9 release leaf spring 7 to interrupt the aforementioned current path between terminal elements or terminal caps 2a, 2b when solder means 9 reaches a predetermined temperature. The position assumed by leaf spring 7 upon being released by solder means 9 is shown in FIG. 1 by dash-dot lines.

Insulating strip 3 is provided adjacent its left end with a perforation into which the axially inner bare end of wire 41') and an eyelet 6a are inserted. The axially inner bare end of wire 4b is bent in the shape of a U. The shank of eyelet 6a projects through a bore 7a in the left end of leaf spring 7, and the fianges of eyelet 6a firmly press leaf spring 7 and wire 4b against insulating strip 3. This establishes a good solderless contact having a negligible voltage drop. The axially inner end of wire 4a is bent in the shape of a U and inserted jointly with an eyelet 6b into a bore situated at the right end 3b of insulating strip 3. The flanges of eyelet 6b clamp wire 4a in position minimizing the voltage drop between eyelet 6b and wire 4a. The hollow spaces inside the shank of eyelet 6b receive solder means 9 which normally maintain leaf spring 7 in its position. Parts 6b and 9 form jointly a solder pot. Due to the capillary action of the shank of eyelet 6b the solder means 9 penetrate deeply into it, and the relatively large mass of solder inside of the shank of eyelet 9 has sufficient heat absorbing capacity to achieve significant delay times. The relatively large amount of heat required to cause softening of solder :means 9 after predetermined time delays are generated immediately adjacent solder means 9 by the cross-over Cr of wires 4a, 4b situated immediately adjacent thereto.

It will be observed that each of eyelets 6a, 6b establishes dual contacts, or two point contacts, with wires or fusible conductor-s 4b and 4a respectively, which tends to minimize the voltage drop between these points and establishes a safety factor against contact failure.

Solder means 9 might be replaced, if desired, by other heat softenable substances capable of normally maintaining leaf spring 7 in current-carrying engagement with eyelet 6b and releasing leaf spring 7 when a predetermined yield temperature is being reached.

Although this invention has been described in considerable detail, it is to be understood that such description of the invention is intended to be illustrative rather than limiting, as the invention may be variously embodied, and is to be interpreted as claimed.

We claim as our invention:

1. In an electric fuse for small cur-rent intensities the combination of j (a) a tubular casing of electric insulating material having a first end and having a second end;

(b) a pair of terminal elements including a first terminal element closing said first end of said casing and a second terminal element closing said second end of said casing;

(c) a strip of insulating material arranged inside said casing substantially in a direction longitudinally thereof, said strip having a first end adjacent said first end of said casing and remote from said second end of said casing, and said strip having a second end adjacent said second end of said casing and remote from said first end of said casing;

(d) a first length of wire extending from said first terminal element to said second end of said strip and being at least in part covered by a first sleeve of insulating lmaterial;

(e) a second length of wire extending from said second terminal element to said first end of said strip .and being at least in part covered by a second sleeve of insulating material;

(f) a leaf spring supported by said strip conductively interconnecting the end of said first length of wire adjacent said second end of said strip and the end of said second length of wire adjacent said first end of said strip, said leaf spring having a first end fixedly secured to said strip and a second end biased away from said strip; and

(g) heat softenable conductive means releasably securing said second end of said leaf spring in position to normally establish a current path from said first terminal element through said first length of wire and through said leaf spring and through said second length of Wire to said second terminal element.

2. An electric fuse as specified in claim 1 wherein (a) said strip of insulating material has a first bore adjacent said first end thereof and has a second bore adjacent said second end thereof, said first length 4 of wire having an end inserted into said second bore and said second length of Wire having an end inserted into said first bore;

(b) a pair of eyelets is supported by said strip, one of said pair of eyelets being inserted into said first bore and clamping in position therein said second length of wire, and the other of said pair of eyelets being inserted into said second bore and clamping in position therein said first length of wire; and

(c) one of said pair of eyelets securing said first end of said leaf spring fixedly to said strip and the inside of said other of said pair of eyelets being filled with said heat softenable means for releasably securing said second end of said leaf spring in position.

3. An electric fuse for small current intensities includ- (a) a tubular casing of insulating material having a left end and a right end;

(b) a left terminal cap mounted on said left end of said casing and a right terminal cap mounted on said right end of said casing and closing said casing;

(c) an oblong strip of electric insulating material arranged inside said casing in a direction substantially longitudinally thereof and having a left end and a right end;

(d) a pair of lengths of fusible conductor including a first length of fusible conductor extending from said left terminal cap to said right end of said strip and covered with a first sleeve of glass cloth, and a second length of fusible conductor extending from said right terminal cap to said left end of said strip and covered with a second sleeve of glass cloth,

(e) a stressed leaf spring conductively interconnecting the ends of said first length of fusible conductor and of said second length of fusible conductor located adjacent said strip and completing a current path from said left terminal cap through said first length of fusible conductor and through said second length of fusible conductor to said right terminal cap; and

(f) solder means normally restraining said leaf spring in the stressed position thereof and releasing said leaf spring to interrupt said current path when said solder means reaches a predetermined temperature.

4. An electric fuse as specified in claim 3 wherein (a) said strip is provided with a pair of perforations one adjacent said left end and one adjacent said right end thereof;

(b) said first length of fusible conductor being threaded through'one of said pair of perforations and said second length of fusible conductor being threaded through the other of said pair of perforations;

(0) a pair of eyelets each being inserted into one of said pair of perforations, one of said pair of eyelets also projecting through a perforation provided in said leaf spring and clamping with a flange portion thereof said leaf spring and one of said pair of length of fusible conductor against said strip; and

(d) the other of said pair of eyelets clamping with a flange portion thereof the other of said pair of length of fusible conductor against said strip, said other of said pair of eyelets containing a mass of soft solder in the shank portion thereof having a predetermined yield temperature and normally maintaining said leaf spring in said stressed position thereof.

5. In an electric fuse for small current intensities the combination of (a) a tubular casing of insulating material;

(b) a pair of terminal caps each mounted on one of the ends of said casing;

(c) an oblong strip of insulating material arranged inside of said casing substantially in a direction longitudinally thereof, said strip having a pair of perforations each adjacent one of the ends thereof;

(d) a first length of wire having an insulating sleeve (f) a stressed leaf spring conductively interconnecting the adjacent ends of said first length of wire and of said second length of wire and completing a current path between said pair of terminal caps including said first length of wire and said second length of wire;

(g) an eyelet inserted into one of said pair of perforations in said strip and extending with the shank portion thereof through a perforation in said leaf spring adjacent one of the ends thereof, the flange portions of said eyelet clamping two points of said first length of Wire against said strip and said flange portions of said eyelet further clamping said leaf spring against said strip; and

(h) another eyelet inserted into the other of said pair of perforations of said strip, the flange portions of said another eyelet clamping'two points of said second length of wire against said strip, and the shank portion of said another eyelet being filled with a mass of soft solder maintaining a point of said leaf spring adjacent the other of the ends thereof in current-carrying engagement with said another eyelet.

10 References Cited UNITED STATES PATENTS 681,495 8/1901 Chantler 337-405 2,300,620 11/ 1942 Dverkos 337-164 15 2,667,551 1/1954 Berthel 337166 2,727,110 12/1955 Von Hoorn 337-165 2,833,890 5/1958 Jacobs 337-290 X 3,122,619 2/1964 Fister 337-464 US. Cl. X.R, 

